It heretofore has been a common practice that when, for example, green ceramic honeycomb structural bodies are to be fired, the structural bodies are fired in a tunnel kiln or a periodic kiln, while being placed on a shelf plate. In order to prevent cracking of the structural body and bonding between the structural body and the shelf plate, underlying boards called setters are interposed between the green ceramic honeycomb structural bodies and the shelf plate during firing.
Japanese patent publication No. 1-54,636 discloses, as an example of the underlying board, a setter made of a ceramic plate with a honeycomb structure and having a burred portion at an upper edge. For example, when a honeycomb structural body composed mainly of cordierite is to be fired, it is a common practice to employ an underlying board made of the same material (100% cordierite) as that of the honeycomb structural body so as to make the shrinkage factor equal between the underlying board and the honeycomb structural body to be fired.
However, in the case of the above-mentioned underlying boards, problems arise as the number of times the board is passed through the tunnel kiln increases. Specifically, the underlying board is warped when heated, and the surface of the structural body becomes coarse due to melting-out of a silica component so that the bottom end face of the honeycomb structural body is deformed, ribs of the honeycomb structural body are twisted and/or the structural body is cracked. Particularly, if the underlying board made of 100% cordierite is used in a tunnel kiln, the above problems such as warping and melting-out of the silica component are conspicuous, because such an underlying board has poor resistance against direct flame from a burner of the tunnel kiln. Owing to this, the conventional underlying boards have the problem that the number of times of passing through the tunnel kiln may be as low as 15-20.